A uniform illumination field is necessary to enable sophisticated image processing techniques to be applied to quantitative microscopy. Achieving this uniformity is possible with a single illumination source, but it becomes tedious if more than one source is used to excite different chromophores within the sample of interest. A 3-to-1 optical coupler was designed and constructed to permit selection of three different sources for introduction into the microscope without disturbing the critically aligned input optical fiber to the microscope illumination port. Sources may be employed in the ultraviolet down to 250nm.